1. Field of the Invention
The invention relates to arc welding, in particular to a consumable-electrode AC gas shield arc welding method and an apparatus therefor.
2. Description of the Related Art
Prior art teaches welding in which DC current is passed through from an electrode (hereinafter referred to as wire) of positive polarity to a base metal of negative polarity in such a way that the average welding current may be controlled by varying the period of the base current while keeping its peak current, peak current period, and base current fixed, as disclosed in Japanese Patent Laid-Open Publication No. Sho. 56-165564. It is then possible to transfer the droplets formed at the tip of the wire towards the molten pool during the peak current period, thereby permitting welding which is less subject to spattering.
In order to obtain improved arc stability for small average welding current, Japanese Patent Laid-Open Publication No. Sho. 59-50672 discloses welding where a first electric pulse transfers the droplet from the wire and a second pulse prevents arc vanishing that follows the burning of the wire on the tip caused by the first pulse.
In a so-called reverse polarity welding where wire is given positive polarity and the base metal negative polarity, a substantially straight arc is established between the wire tip and a point of the surface (cathode spot) of the base metal just beneath the wire tip in the case of great welding current, while the cathode spot, and hence the arc itself, widely rambles on the surface of the base metal in the case of small welding current, so that the arc might vanish if it extends too long.
For these reasons the average welding current cannot be made smaller than a certain threshold in the said first prior art, nor said arc vanishing could not be prevented sufficiently in the said second prior art.
Japanese Patent Laid-Open Publication No. Sho. 57-130770 discloses a method for transferring the droplet to the molten pool in synchronism with the peak of the welding current which is passed through from a positive electrode (wire) to a negative base metal and is periodically varied between a maximum (peak current) and a minimum (base current).
For further understanding it would be appropriate to describe here a general relationship between the polarity of wire and the behaviors of arc. When the wire is given positive polarity, fairly immobile positive end of the arc is formed on the lower end of the droplet formed on the wire tip. As the result the arc is well bundled.
On the other hand if the wire is given negative polarity a diverging arc is formed from the droplet since mobile negative ends of the arc are then formed over the entire droplet surface. Therefore, when the polarity of the wire is changed from plus to minus, the arc pressure decreases as shown in FIG. 14 from curve a to curve b, so that the melting of the melting of the base metal is suppressed. However, if the wire is held minus only, much too large droplet is formed, since the amount of wire melt is governed by the polarity of the wire as depicted in FIG. 15, and hence the arc becomes unstable.
It should be noted that the use of AC current, permitting alternating polarities of plus and minus for the wire, may provide a stable arc configuration while suppressing melting of the base metal.
In a so-called reverse polarity welding where the wire is held positive and the base metal negative, the arc exhibits a greater pressure compared with positive polarity welding where the wire is held negative and the base metal positive, and consequently causes deeper penetration in the base metal and possible melting down of the metal if it is a thin metal plate. This is the case for metals like aluminum having low melting points.
In MIG braze welding using copper wire, excessive melting of the base metal must be avoided with great care since it causes the infiltration of copper into the base metal and results in weld cracks.
Suppression of the melting of the base metal is also desirable to reduce the dilution rate in the case of build-up welding of different metals.
In Japanese Patent Laid-Open Publication No. Hei. 1-186279 proposes a consumable electrode gas shield AC arc welding method and means therefor in which AC frequency is chosen appropriately chosen for a given wire feeding rate, along with a period and a level of reverse polarity current suitable for the shield gas and the material properties and diameter of the wire used, and levels of the normal and reverse polarity currents are controlled based on the arc voltage detected to keep the arc length constant.
The prior art welding mentioned above, however, has drawbacks described below. The prior art welding assumes AC rectangular waves, which are not in actuality perfectly rectangular as shown by a dotted waveform in FIG. 21 but rather trapezoidal as shown by a solid waveform. This is due to the fact that in actual welding processes the inductances and resistances of a power supply cable, usually 10 to 20 m long, affect the waveforms. In FIG. 21 the coordinate and abscissa represent current and time, respectively. The figure shows that the arc polarity is straight when the welding wire is negative (EN), while the polarity is reverse when the welding wire is positive (EP). I.sub.EP is the level or height of the rectangular current wave in the reverse polarity; T.sub.EP, its duration, while I.sub.EN is the height of the wave in the straight polarity; T.sub.EN, its duration, where I.sub.EP &gt;I.sub.EN is assumed.
It is not possible with such trapezoidal waves to maintain AC current if the period T.sub.EN is shortened and the AC frequency is increased so as to meet the requirement of increasing wire feeding rate, since the time interval required for polarity change is not secured then, resulting in DC current as shown in FIG. 22. Furthermore, the period T.sub.EN will lose its constancy as the current changes from AC to DC, since the slopes of the trapezoidal wave are affected by the length and the arrangement of the welding cable used.
Furthermore, under a welding control method in which arc length is to be held constant as in prior art mentioned above, the current waveform changes as shown in FIGS. 23 and 24. Namely, if the arc length becomes shorter the electric current is increased to melt more wire so as to restore arc length and if the arc length becomes too long the current is decreased to melt less wire so as to restore the arc length, so that the amplitude of the current varies with the arc length. Consequently, AC Periods are eventually mingled with DC periods as shown in FIG. 25 as the period T.sub.EN of the wave having straight polarity is shortened.
Since AC and DC configurations are utterly different, coexistence of AC and DC arcs will result in fluctuations in penetration and melting of the wire, causing weld flaws such as overlaps and lack of fusion. Further, sudden change in arc configuration would upset welders and greatly reduce welding operability.
In non-consumable electrode AC gas shield arc welding as disclosed in Japanese Patent Laid-Open Publication No. Hei. 1-100672, a polarity EP (in which the electrode is negative) is maintained until an arc is initiated, but the current is switched from DC to AC immediately after the arc is initiated. This publication does not deal with welding whose arc is initiated by contacting the electrode on the base metal, since a high-frequency AC current is used in the non-consumable electrode AC welding method.
In a commercial consumable electrode DC gas shield arc welding apparatus an output power for arc initiation is set greater than that of normal operating output level.
Although such a consumable electrode DC welding as mentioned above deals with arc initiation with the consumable electrode in contact with the base metal, it is restricted to DC welding and no polarity switching is employed.
It should be noted that, because the penetration is smaller in EN polarity than in EP polarity, the switching of wire polarity from EP to EN immediately after the initiation of arc is likely to cause weld flaws and/or overlaps in the initiating arc region. Further, since the current is once reduced to zero and raised in opposite polarity, such polarity switching may destroy the initiating arc once and restart an arc, making an inherently unstable initiating arc more unstable.